1. Technical Field of the Invention
This invention relates generally to mixed signal integrated circuits and more particularly to multiple independent outputs of a system on a chip.
2. Description of Related Art
In general, a system on a chip (SOC) integrates multiple independent circuits, which are typically available as individual integrated circuits, onto a single integrated circuit. For example, an audio processing SOC combines a processing core (e.g., microprocessor and/or digital signal processor, instruction cache, and data cache), an audio codec (e.g., digitization of analog audio input signals and converting digitized audio signals into analog output signals), a high speed serial interface (e.g., universal serial bus (USB) interface), and an external memory interface.
To facilitate the conversion of digitized audio signals into analog output signals, the audio codec of an audio processing SOC includes a digital to analog converter (DAC) that provides its output to a headphone amplifier and/or to a line out driver, but with limitations. For example, in one known embodiment, the line out driver and the headphone amplifier are serially coupled to output of the DAC. In this embodiment, the headphone amplifier is dependent on line out driver such that the line out driver cannot be muted and/or powered down without affecting the headphone amplifier.
In another known embodiment, the headphone amplifier and line out driver are separately coupled to the output of the DAC. In this embodiment, the DAC provides volume control for the headphone amplifier, which also affects the signal level to the line out driver. To provide a relatively constant signal level output, the line out driver includes an inverse volume control function to counteract the volume adjustments by the DAC, which has limited accuracy. While this embodiment provides digital volume control and a wider volume range via the DAC, it adversely affects the line out driver.
In either of the above embodiments, the DAC, the headphone amplifier, and the line out driver are supplied with the same voltage. The typical output levels of the line out driver mandate the use of a higher supply voltage than what is necessary for the headphone amplifier and the DAC. Using a common voltage supply can save pins by sourcing the three circuits through a single pin. This comes with the cost of the extra power consumed by running the DAC and the headphone amplifier at higher than needed supply voltages.
In addition, the DAC, the headphone amplifier, and the line out driver have separate ground pin connections to provide isolation between the circuits. While this provides a desired level of isolation, it requires extra pins to implement.
Therefore, a need exists for a SOC that includes multiple independent outputs that overcomes one or more of the above mentioned limitations.